Image capturing devices, such as cameras using photographic film, video cameras using video tape, and digital cameras using an array of charge coupled devices in a semiconductor typically, all use some optical system to focus light from an object onto the image capturing medium, such as photographic film or an array of charge coupled devices (CCD). Various types of optical systems exist in the prior art. In each of these types, there is typically a view-finder which allows the photographer or user of the camera or other image capturing device to observe the view being recorded (or to be recorded) on the image capturing medium. Some of these view-finders see almost exactly the same field of view and frame of view as the image capturing medium; an example of this type of optical system is the classic single lens reflex camera. In this camera, a mirror behind the lens directs light from the image which comes through the lens to the viewing screen of the view-finder, and this mirror is removed during exposure of the film in order to allow light from the image to reach the film. Other types of image capturing devices such as view-finder cameras have one lens system to direct the light from the image to the image capturing medium and another lens system (e.g. the view-finder) to provide a view for the user of the camera. In these view-finder systems, the view displayed in the view-finder does not exactly match the view appearing on the image capture medium, such as the photographic film or CCD. This results from an effect known as parallax.
FIG. 1 shows an example in the prior art of a fixed focus view finder camera 100. View finder camera 100 is a digital camera, an embodiment of which is known as the QuickTake 100 camera from Apple Computer, Inc. of Cupertino, Calif. The front 102 of this camera includes a primary lens 103 which focuses light from a subject onto the CCD image capture medium. The camera 100 also includes a view-finder lens 104 which focuses light from the subject into the view-finder so that the camera user's eye can focus upon the image/subject being viewed. The camera 100 also includes a light source 105, which is typically a flash which is controlled by a sensor 106 which monitors the amount of the flash being produced to illuminate the subject being recorded.
FIG. 2 illustrates the optical system of the camera 100. The optical system is designed such that, with a fixed focus camera, all objects from approximately 1.1 meters to infinity from the camera are in focus. This distance of 1.1 meters is shown as d.sub.1. As can been seen from FIG. 2, an optical parallax occurs by virtue of the fact that the view-finder 203 is physically displaced away from the primary lens 103 and the image capturing medium 201, such as CCD or photographic film. This physical displacement causes the view through the view-finder to not exactly match the view of the image appearing on the image capturing medium 201. This may be seen from FIG. 3, which shows two view cones 302 and 303; one cone 302 extends from the primary lens and the other cone 303 extends from the view-finder lens 104. The optical axis (also know as center axis) of each lens bisects the view cone which extends from the lens. The distance at which the optical axis of the view-finder lens intersects the optical axis of the primary lens is about 2 meters from the camera. At this distance, the views (frame) of the primary lens and the view-finder substantially coincide. At large distances (e.g. 10 meters) away from the camera, the parallax problem is not significant due to the convergence at d.sub.1 of the view cones 302 and 303 shown in FIG. 3. Thus, at large distances the eye 202 of the user looking through the view finder 203 (which includes a view finder lens 104 and a view finder lens 211) sees the image in substantially the same field as is imaged upon the image capturing medium 201.
However, at short distances between the subject being photographed and the camera, the parallax problem becomes significant such that the overlap 301 of the view cones 302 and 303 is not sufficient. This can be seen in FIG. 3 when the camera is being used in a "close-up" view where the distance d.sub.2 is considerably smaller than the distance d.sub.1. In the example shown in FIG. 3, d.sub.2 may be, for example, one foot while d.sub.1, which represents the minimum distance at which objects are in focus may be, for example, about 4 feet. At a close-up range the camera has several deficiencies which must be corrected. The use of fixed focus view-finder cameras in close-up work has always been difficult due the parallax problem and also due to the lack of focus control. Typically, these types of cameras cannot focus this close and thus the image will always be out of focus unless the optics are adjusted. The corrective optics must also take into account the severe parallax problem at such short distances, such as one foot between the subject being recorded and the camera. A corrective optic element must be placed in front of the primary lens 103 in order to focus the image at a close-up range, such as at d.sub.2, upon the image capturing medium 201. Also, a corrective optic element, such as an optical lens must placed in front of the view-finder lens 104 in order to allow the subject at d.sub.2 to appear properly framed to the eye 202 of the user. That is, the parallax problem at short distances between the view-finder and the primary lens is corrected with the corrective lens which is placed in front of the lens 104. Also, it is often necessary to redirect, attenuate and diffuse the light from the flash unit 105 so that the subject in the close-up range at d.sub.2 is not illuminated with too much light or light that has "hot spots" which are brighter than other spots on the subject.
The prior art includes a lens assembly (or lens adapter) which is attached to a camera to convert a fixed focus view finder camera into a close-up range camera. For example, Ace Optical of Japan has provided lens assemblies containing a corrective lens for the primary lens of the camera as well as a corrective lens for the view-finder lens of the camera. This lens assembly then allows the camera to be used at a close-up range by modifying the field of view through the view finder and the focus through the primary lens.
These corrective lens assemblies by themselves do not, however, correct for another problem which occurs when using such a camera at short close-up ranges. This problem, depth of field, means that the positioning of the camera relative to the subject being recorded is critical in order to ensure that the subject is properly in focus. At close-up ranges, an optically corrected fixed focus camera tends to have a very small depth of field; typically, this depth of field is about 2 inches, meaning that only those objects within 1 inch of the correct focusing distance will be in focus. Because the eye, viewing the image through the view-finder, cannot ascertain whether the image is focused on the image capturing medium, it becomes impossible for the user of such a camera with a corrective lens assembly to accurately judge when the image is focused at a close-up range on the image capturing medium. One solution in the prior art includes using a measuring stick or a measuring string which extends from the front (or other portion) of the camera or from devices attached to the camera and is designed in length to measure exactly the distance where the image will be in focus. So, for example, the camera shown in FIG. 2 or in FIG. 1 may be fitted with the corrective lens assembly for allowing the camera to be used at close-up range, and this corrective lens assembly may include a string having a defined length so that the user may place the camera relative to the subject to be photographed at the proper distance such that the subject is focused on the image capturing medium 201.
While this solution to the depth of field problem is adequate in some circumstances, it is desirable to provide an improved solution which does not require the user to carry around a measuring device.